Plasma generation method and apparatus

ABSTRACT

A plasma generation apparatus and method, which achieve both sterilization and deodorization of attached bacteria even under the condition that steam or fine droplets of water are present. A pair of electrodes is prepared, plasma discharge is carried out by applying designated voltage between the pair of electrodes, fluid passage holes are provided at corresponding parts of respective electrodes so as to communicate with each other, and steam or fine droplets of water are applied to the fluid passage holes and plasma generated around the fluid passage holes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 13/312,253, filed Dec. 6, 2011, which claims the priority benefit ofJapanese Patent Application No. 2010-273450, filed on Dec. 8, 2010 inthe Japanese Patent Office, the disclosures of each of which areincorporated herein by reference.

BACKGROUND

1. Field

Example embodiments of the following description relate to a plasmageneration method and apparatus, and more specifically, to a method andapparatus for sterilization and deodorization of attached bacteria underthe condition that steam or fine droplets of water are present.

2. Description of the Related Art

Recently, the requirements of air quality control in livingenvironments, such as, sterilization or deodorization, have become morestringent due to the increase in individuals suffering from atopy,asthma, and allergies, or the increase in the risk of infectiousdiseases, represented by the explosive spread of new types of influenza.Further, as society becomes increasingly affluent, the amount of storagefood increases or an opportunity to store leftovers increases, and thus,the importance of controlling an environment in the storage equipment,such as a refrigerator, increases.

In order to control air quality in living environments, physical controlgenerally using a filter was conventionally executed. Through physicalcontrol, relatively large dust floating in air and/or bacteria orviruses may be trapped according to sizes of filter holes. Further,activated carbon having anhydrous absorption sites may trap odorousmolecules. However, since, in order to trap such substances, air in aspace of a target object to be controlled needs to completely passthrough the filter, the size of the apparatus must be large, themaintenance cost required to replace the filter is increased, and theapparatus has no effect upon attached particles. Therefore, in order toachieve sterilization and deodorization of the attached particles,chemical active species may be discharged to a space desired to besterilized or deodorized. When a medicine is distributed, or an aromaticor a deodorizer is discharged, active species need to be prepared inadvance and periodic supplement of the active species is indispensable.On the other hand, units which generate plasma in the atmosphere andattempt sterilization and deodorization, using chemically active speciesgenerated, due to plasma generation, have recently been increasinglyproposed.

Technology for generating plasma in the atmosphere through discharge andachieving sterilization and deodorization by ions or radicals(hereinafter, referred to as active species) generated thereby, may beclassified into two types, described below.

(1) A passive plasma generation apparatus, which causes bacteria orviruses floating within the atmosphere (hereinafter, referred to asfloating bacteria) or odorous materials (hereinafter, referred to asodors) to react with active species within a restricted volume in theapparatus (for example, Japanese Patent Laid-open Publication No.2002-224211); and

(2) An active plasma generation apparatus, which discharges activespecies generated by a plasma generation unit into a closed space (forexample, a living room, a bath room, the inside of a car, etc) having alarger volume than the passive plasma generation apparatus, and causesthe discharged active species to react with floating bacteria or odorsdue to collision with the floating bacteria or odors in the atmosphere(for example, Japanese Patent Laid-open Publication No. 2003-79714).

The passive plasma generation apparatus may be advantageous in that highsterilization and deodorization of the air is expected throughgeneration of active species of a high concentration due to generationof plasma in a small volume. However, the passive plasma generationapparatus may be disadvantageous in that the floating bacteria or odorsneed to be introduced into the apparatus, and thus, the size of theapparatus is increased, ozone as a byproduct due to plasma generation isgenerated, and separate installation of a filter to absorb or decomposeozone to prevent ozone from leaking to the outside of the apparatus isrequired.

In addition, the active plasma generation apparatus may be advantageousin that the size of the apparatus is relatively reduced, andsterilization of bacteria attached to the surfaces of clothes orhousehold goods (hereinafter, referred to as attached bacteria), ordecomposition of odors absorbed to the surfaces, is expected in additionto sterilization of floating bacteria or decomposition of odors in air.However, the active plasma generation apparatus may be disadvantageousin that the concentration of active species is decreased due todiffusion of the active species into a considerably large closed space,as compared with the volume of the apparatus, and thus, sterilizationand deodorization are expected only upon active species having a longlife. Consequently, deodorization effects are scarcely expected in aspace having a high concentration of odors (concentration 10,000 timesgreater than the concentration of the active species).

As described above, the passive plasma generation apparatus exhibitseffects restricted to floating bacteria or odors contained in an airflow introduced into the apparatus, and the active plasma generationapparatus exhibits effects upon floating bacteria, attached bacteria,and odors having a low concentration. Further, conventional plasmageneration apparatuses may be disadvantageous in that an amount ofgenerated ions or radicals is reduced due to lowering of the performanceof the plasma generation apparatus in a high-humidity state. That is,the conventional plasma generation apparatuses may achieve eithersterilization of floating bacteria and deodorization in a high-humiditystate, or sterilization of floating bacteria and attached bacteriahaving a low concentration and deodorization of attached odors having alow concentration in a high-humidity state.

However, there are several situations in daily life in which bothsterilization of attached bacteria and deodorization of odors of a highconcentration are desired to be simultaneously achieved under thecondition that steam or fine droplets of water are present in ahigh-humidity environment. Typically, there are home appliances fortreating water. For example, a washing machine is generally in a highhumidity state, various attached bacteria are present on the surface ofa tub, and odors generated due to remaining water or decomposition of adetergent occur within the washing machine. Further, various bacteria orodors generated due to remaining food waste may be present within a dishwasher.

SUMMARY

The foregoing and/or other aspects are achieved by providing a plasmageneration apparatus and method, which achieve both sterilization anddeodorization of attached bacteria, for example, under the conditionthat steam or fine droplets of water are present in a high-humidityenvironment, and particularly, a plasma generation apparatus and methodwhich increase an amount of generated active species and exhibit both anactive function of generating functional mist by interaction between theactive species and steam or fine droplets of water to achievedeodorization and a passive function of discharging functional mist tothe outside of the apparatus to sterilize attached bacteria.

The foregoing and/or other aspects are also achieved by providing aplasma generation method, in which a pair of electrodes is prepared andplasma discharge is carried out by applying designated voltage betweenthe pair of electrodes, fluid passage holes are provided atcorresponding parts of respective electrodes so as to communicate witheach other, and steam or fine droplets of water are applied to the fluidpassage holes and plasma generated around the fluid passage holes. Inthis case, the corresponding parts mean that the respective fluid holesformed on the electrodes are located at the same positions as seen fromthe plane directions of the electrodes, and are located at approximatelythe same x and y coordinate positions of the electrodes as the pair ofelectrodes on an x-y plane is observed in the z axis direction in arectangular coordinate system.

Through the above configuration, an amount of plasma generated from therespective corresponding fluid passage holes may be increased and acontact area between the plasma and a fluid may be increased, therebyincreasing an amount of generated active species. Further, a contactarea between the plasma and the steam or fine droplets of water may beincreased. The steam or fine droplets of water may contact the activespecies generated from the fluid passage holes, and thus, the activespecies may charge or be mixed with the steam or fine droplets of water,become functional mist, and be discharged to the outside. The functionalmist discharged to the outside may sterilize flowing bacteria andattached bacteria. Moreover, the amount of the active species generatedfrom plasma may be increased, thereby exhibiting a sufficientdeodorizing function.

The foregoing and/or other aspects are also achieved by providing aplasma generation method, in which a pair of electrodes is prepared andplasma discharge is carried out by applying designated voltage betweenthe pair of electrodes, wherein through holes are provided on oneelectrode such that openings of the through holes on an opposite surfaceof the electrode facing the other electrode are closed by the otherelectrode, and steam or fine droplets of water are applied to openingsof the through holes on the other surface of the electrode and plasmagenerated around the through holes.

Through the above configuration, an amount of plasma generated from therespective corresponding fluid passage holes may be increased and acontact area between the plasma and a fluid may be increased, therebyincreasing an amount of generated active species. Further, a contactarea between the plasma and the steam or fine droplets of water may beincreased. Thereby, an amount of generated functional mist may beincreased, thus sterilizing flowing bacteria and attached bacteria.Moreover, the amount of the active species generated from plasma may beincreased, thereby exhibiting a sufficient deodorizing function.

An embodiment of the present disclosure provides that the fine dropletsof water may have a particle size of approximately 100 μm or less. Thereason why the fine droplets of water have a particle size ofapproximately 100 μm or less is that a distance between the electrodesis approximately 100 μm or less, and thus, steam or fine droplets ofwater having a particle size of more than approximately 100 μm are notbe applied to the electrodes. Further, droplets of water having aparticle size exceeding approximately 100 μm form, for example, a waterdrop shape like rain, are not contained in a fluid, and fall by gravity,thus not generating functional mist.

In order to increase the number of the active species contained in thefluid having passed through the fluid passage holes to increase theamount of the generated functional mist and to suppress concentration ofgenerated ozone, voltage in a pulse mode may be applied to therespective electrodes and may have a peak value within the range ofapproximately 100V to 5,000V and a pulse width in the range ofapproximately 0.1 μs to 300 μs.

Another embodiment provides that the steam or fine droplets of water maybe supplied when a temperature of the pair of electrodes is more than adesignated value. That is, if the temperature of the pair of electrodesis more than the designated value, it may be judged that an amount ofmoisture in the atmosphere and an amount of moisture attached to theelectrodes is lowered, and thus, the steam or fine droplets of water maybe supplied.

Further, the pair of electrodes may be heated by a heating unit when thetemperature of the pair of electrodes is less than the designated value.Thereby, since it is judged that the amount of moisture attached to theelectrodes is excessively large, the electrodes may be heated toevaporate the moisture attached to the electrodes, without supply of thesteam or fine droplets of water. Accordingly, performance of theelectrodes is not lowered due to moisture, and the steam or finedroplets of water may be applied to the fluid passage holes at alltimes.

The foregoing and/or other aspects are also achieved by providing aplasma generation apparatus, which includes a pair of electrodesprovided with a dielectric film on at least one of opposite surfacesthereof, a voltage application unit to apply designated voltage betweenthe pair of electrodes to carry out plasma discharge, and fluid passageholes provided at corresponding parts of respective electrodes so as tocommunicate with each other, a fluid contacting the plasma to generateions or radicals when the fluid passes through the fluid passage holes,wherein a supply device to supply steam or fine droplets of water to thefluid passage holes or plasma generated around the fluid passage holesis provided.

In this case, since the dielectric film is provided on at least oneelectrode among the pair of electrodes, a gap to generate plasma may beformed between the respective electrodes without a spacer to form thegap.

In order to increase a contact area between a fluid passing through thefluid passage holes and the plasma in order to increase an amount ofgenerated functional mist, at least parts of the outlines of therespective fluid passage holes, corresponding to each other, may belocated at different positions as seen from the plane direction of theelectrodes.

In order to locate the at least parts of the outlines of the respectivefluid passage holes corresponding to each other at different positions,an opening size of the fluid passage holes formed on one electrode amongthe pair of electrodes may be smaller than an opening size of the fluidpassage holes formed on the other electrode by approximately 10 μm ormore. In addition, the fluid passage holes having the same opening sizemay be disposed such that the centers of the openings thereof do notmatch each other.

In order to increase an amount of discharged active species to achievedeodorization of the fluid passing through or having passed through thefluid passage holes, and sterilization of floating bacteria contained inthe fluid to increase an amount of generated functional mist, throughholes may be provided on one electrode separately from the fluid passageholes and openings of the through holes on an opposite surface of theelectrode facing the other electrode may be closed by the otherelectrode. The fluid, after having passed through the fluid passageholes, and the steam or fine droplets of water may be introduced intothe through holes to contact the plasma, or the fluid prior to passingthrough the fluid passage holes and the steam or fine droplets of watermay be introduced into the through holes to contact the plasma, therebyremarkably improving effects of the plasma generation apparatus.

An example embodiment provides that an opening size of the through holesmay be smaller than the opening size of the fluid passage holes byapproximately 10 μm or more.

Another example embodiment provides that the dielectric film may have asurface roughness of more than approximately 0.1 μm and less than 100μm. Thereby, even if the pair of electrodes is stacked without use of aspacer, a space in which the plasma is generated may be formed by thesurface roughness.

In order to allow the fluid to effectively pass through the fluidpassage holes to promote generation of active species and to increasedeodorizing effect, the plasma generation apparatus may further includean air blowing device to forcibly blow air toward the fluid passageholes.

Another example embodiment provides that a flow velocity of air blown bythe air blowing device and passing through the fluid passage holes maybe in the range of approximately 0.1 m/s to 10 m/s. When the flowvelocity of air is less than approximately 0.1 m/s, efficiency of thefluid passing through the fluid passage holes is low, and when the flowvelocity of air is less than approximately 10 m/s, reaction between thefluid and the plasma is sufficiently carried out.

In order to prevent lowering of performance of the pair of electrodesdue to moisture, a water repellent property may be imparted to theopposite surface of at least one electrode, among the pair ofelectrodes, so that a contact angle of water drops with the oppositesurface is more than 90 degrees. In order to uniformly maintain appliedvoltage to keep a large amount of generated active species, even if anexcessive amount of moisture contacts the plasma electrode unit, theopposite surfaces of the electrodes have the water repellent property.

If a conventional plasma generation apparatus requiring high voltage isapplied to a refrigerator using a combustible gas instead of Freon gas,safety may be lowered. The used combustible gas may leak in therefrigerator, and sparks causing high voltage which are generated in theabove atmosphere may ignite and cause an explosion accident. Therefore,the plasma generation apparatus may further include an explosion proofdevice, including protective covers disposed at the outside of the pairof the electrodes to prevent flame generated from the combustible gasintroduced into the fluid passage holes by the plasma from propagatingto the outside over the protective covers.

In order to obtain safety without lowering deodorizing and sterilizingcapacities, each of the protective covers may include a metal meshdisposed at the outside of the pair of the electrodes, and the metalmesh may have a wire diameter of approximately 1.5 mm or less and anaperture ratio of approximately 30% or more.

Additional aspects, features, and/or advantages of example embodimentswill be set forth in part in the description which follows and, in part,will be apparent from the description, or may be learned by practice ofthe disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects of the present disclosure will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 is a perspective view illustrating a plasma generation apparatusin accordance with example embodiments;

FIG. 2 is a schematic view illustrating operation of the plasmageneration apparatus, according to example embodiments;

FIG. 3 is a plan view illustrating an electrode unit, according toexample embodiments; FIG. 4 is a cross-sectional view illustrating theelectrode unit and an explosion proof device, according to exampleembodiments;

FIG. 5 is an enlarged cross-sectional view illustrating configuration ofopposite surfaces of the electrode unit, according to exampleembodiments;

FIGS. 6A and 6B are a partially enlarged plan view and a sectional viewschematically illustrating fluid passage holes and through holes,according to example embodiments;

FIG. 7 is a graph illustrating ionized water densities per unitcircumferential length (ratios to a symmetrical type) according toelectrode shapes, according to example embodiments;

FIG. 8 is a graph illustrating ionized water densities per unitcircumferential length (ratios to the symmetrical type) according to thethrough holes, according to example embodiments;

FIG. 9 is a graph illustrating pulse width dependency of ionized waterdensity and ozone concentration, according to example embodiments;

FIG. 10 is a graph illustrating relations between relative humidity andionized water density, according to example embodiments;

FIG. 11 represents photographs illustrating results of a sterilizationtest of colon bacilli;

FIG. 12 is a sectional view illustrating plasma generation anddeodorization fields, according to example embodiments;

FIG. 13 is a schematic view illustrating sterilization by dischargedactive species and functional mist, according to example embodiments;

FIG. 14 is a schematic view illustrating ignition by plasma andprevention of flame propagation by an explosion proof device duringmalfunction, according to example embodiments;

FIG. 15 is a view illustrating a parameter region of a metal meshsatisfying an explosion proof capacity and ion discharge performance,according to example embodiments; and

FIG. 16 is a sectional view illustrating configuration of an electrodeunit with a heating unit, according to example embodiments.

DETAILED DESCRIPTION

Reference will now be made in detail to the example embodiments of thepresent disclosure, examples of which are illustrated in theaccompanying drawings, wherein like reference numerals refer to likeelements throughout.

A plasma generation apparatus 100 in accordance with an exampleembodiment of the present disclosure is used in a home appliance, forexample, a refrigerator, a washing machine, a laundry dryer, a cleaner,an air conditioner or an air cleaner, particularly, even in ahigh-humidity environment, and is employed as a sterilization anddeodorization apparatus, which achieves deodorization of air at theinside or outside of the home appliance, or sterilization of floatingbacteria or attached bacteria at the inside or outside of the homeappliance.

In more detail, as shown in FIGS. 1 and 2, the plasma generationapparatus 100 includes a plasma electrode unit 2 to generate activespecies, such as ions or radicals, through micro gap plasma, an airblowing device 3 provided at the outside of the plasma electrode unit 2to forcibly blow air (an air current) to the plasma electrode unit 2, afine droplet supply device 4 to supply fine droplets of water (mist) tothe plasma electrode unit 2, an explosion proof device 5 provided at theoutside of the plasma electrode unit 2 to prevent flame generated by theplasma electrode unit 2 from propagating to the outside, and a powersupply 6 to apply high voltage to the plasma electrode unit 2.

Hereinafter, the respective components 2-6 will be described withreference to the accompanying drawings.

The plasma electrode unit 2, as shown in FIGS. 2 to 6B, includes a pairof electrodes 21 and 22 provided with dielectric films 21 a and 22 a onopposite surfaces thereof, and designated voltage is applied between theelectrodes 21 and 22, thereby executing plasma discharge. The respectiveelectrodes 21 and 22, particularly as shown in FIG. 3, have anapproximately rectangular shape as seen from the top (the planedirection of the electrodes 21 and 22), and are formed of, for example,stainless steel, such as SUS 403. Application terminals 2T to whichvoltage from the power supply 6 is applied are formed at the edges ofthe electrodes 21 and 22 of the electrode unit 2 (with reference to FIG.3). Here, in a voltage application method of the plasma electrode unit 2by the power supply 6, voltage in a pulse mode applied to the respectiveelectrodes 21 and 22, a peak value of the voltage is within the range of100 V to 5,000 V, and a pulse width of the voltage is within the rangeof 0.1 μs to 300 μs.

Further, as shown in FIG. 5, the dielectric films 21 a and 22 a areformed on the opposite surfaces of the electrodes 21 and 22 by applyinga dielectric, for example, barium titanate, to the opposite surfaces ofthe electrodes 21 and 22. Surface roughness (arithmetic averageroughness Ra in this embodiment) of the dielectric films 21 a and 22 ais more than 0.1 μm and less than approximately 100 μm. Otherwise,maximum height Ry and ten-point average roughness Rz may be used as thesurface roughness. By restricting the surface roughness of thedielectric films 21 a and 22 a to a value within the above range, a gapis formed between the opposite surfaces of the electrodes 21 and 22 onlyby stacking the electrodes 21 and 22, and plasma is formed within thegap. Thereby, a spacer to form the gap to form plasma is not requiredbetween the respective electrodes 21 and 22. Further, control of thesurface roughness of the dielectric films 21 a and 22 a by a thermalspray process is considered. As a non-limiting example, the dielectricapplied to the electrodes 21 and 22 may use aluminum oxide, titaniumoxide, magnesium oxide, strontium titanate, silicon oxide, silverphosphate, lead zirconate titanate, silicon carbide, indium oxide,cadmium oxide, bismuth oxide, zinc oxide, iron oxide, or carbonnano-tubes.

In order to prevent lowering of performance of the plasma electrode unit2 due to moisture, a water repellent property is imparted to theopposite surface of at least one of a pair of the electrodes 21 and 22so that a contact angle of water drops with the opposite surface is morethan 90 degrees. In this embodiment, in order to uniformly maintainapplied voltage and to keep a large amount of generated active species,even if an excessive amount of moisture contacts the plasma electrodeunit 2, the water repellent property is imparted to the oppositesurfaces of the electrodes 21 and 22, i.e., the surfaces of thedielectric films 21 a and 22 a of the electrodes 21 and 22, as shown inFIG. 5. In order to impart the water repellent property, for example, awater repellent thin film 2M formed of a water repellent material isformed by applying a fluorine-based resin mixed solvent to thedielectric films 21 a and 22 a of the electrodes 21 and 22 to a thinthickness and drying the resin. Such a water repellent thin film 2M isexposed at a plurality of regions on the dielectric films 21 a and 22 agenerating plasma, and repels water without change of a plasmageneration state.

Further, as shown in FIGS. 3, 4 and 6, fluid passage holes 21 b and 22 bare provided at corresponding parts of the respective electrodes 21 and22 so as to communicate with each other, and are configured such that atleast parts of the outlines of the fluid passage holes 21 b and 22 b arelocated at different positions as seen from the plane direction of theelectrodes 21 and 22 (as seen from the top). That is, the shape of thefluid passage holes 21 b formed on one electrode 21 as seen from the topand the shape of the fluid passage holes 22 b formed on the otherelectrode 22 as seen from the top are different.

In more detail, the fluid passage holes 21 b and 22 b formed at thecorresponding parts of the respective electrodes 21 and 22 have anapproximately circular shape as seen from the top (with reference toFIGS. 3, 6A and 6B), and an opening size (an opening diameter) of thefluid passage holes 21 b formed on the electrode 21 is smaller than anopening size (an opening diameter) of the fluid passage holes 22 bformed on the electrode 22 by, for example, 10 μm or more.

Further, as shown in FIGS. 3, 6A and 6B, the fluid passage holes 21 bformed on the electrode 21 and the fluid passage holes 22 b formed onthe electrode 22 are concentric. Further, in this embodiment, all of theplural fluid passage holes 21 b formed on the electrode 21 have the sameshape, all of the plural fluid passage holes 22 b formed on theelectrode 22 have the same shape, and the fluid passage holes 21 bformed on the electrode 21 are smaller than the fluid passage holes 22 bformed on the electrode 22. Although this embodiment illustrates thefluid passage holes 21 b and 22 b as having an approximately circularshape, the fluid passage holes 21 b and 22 b are not limited to thecircular shape as long as at least parts of the outlines of the fluidpassage holes 21 b and 22 b are located at different positions as seenfrom the top.

Further, as shown in FIGS. 3, 6A and 6B, the plasma electrode unit 2 isconfigured such that through holes 21 c are provided on one electrode 21separately from the fluid passage holes 21 b and 22 b and openings ofthe through holes 21 c on the opposite surface of the electrode 21 areclosed by the other electrode 22. The fluid passage holes 21 b and 22 bformed on the respective electrodes 21 and 22 are referred to ascomplete opening parts, and for comparison with the complete openingparts, the through holes 21 c are referred to as half opening parts.

An opening size of the through holes 21 c is smaller than the openingsize of the fluid passage holes 21 b by approximately 10 μm or more. Thethrough holes 21 c are formed as substitutes for some of the regularlyprovided fluid passage holes 21 b, and are provided around the fluidpassage holes 21 b (with reference to FIG. 3).

The air blowing device 3 is disposed on the other electrode 22 of theplasma electrode unit 2, and is provided with an air blowing fan whichforcibly supplies air toward the fluid passage holes (complete openingparts) 21 b and 22 b formed on the plasma electrode unit 2. In moredetail, a flow velocity of air blown by the air blowing device 3 andpassing through the fluid passage holes 21 b and 22 b is in the range ofapproximately 0.1 m/s to 10 m/s.

The fine droplet supply device 4 is disposed, for example, on theelectrode 22 of the plasma electrode unit 2, as shown in FIGS. 1 and 2.The fine droplet supply device is a mist generator which supplies finedroplets of water having a particle size of approximately 100 μm or lessbetween the plasma electrode unit 2 and the air blowing device 3. Thatis, the mist generator is disposed to supply fine droplets of waterupstream of an air flow from the plasma electrode unit 2. In this case,the reason why the fine droplets of water have a particle size ofapproximately 100 μm or less is that a distance between the electrodes21 and 22 is approximately 100 μm or less and steam or fine droplets ofwater having a particle size of more than approximately 100 μm areexcessively large, and thus, do not affect the plasma electrode unit 2.Further, droplets of water having a particle size exceedingapproximately 100 μm form, for example, having a water drop shape likerain, are not contained in a fluid and fall by gravity, thus not movingtoward the plasma electrode unit 2.

The explosion proof device 5 includes protective covers 51 disposed atthe outside of the pair of the electrodes 21 and 22, as shown in FIG. 4,and prevents flame generated from a combustible gas, introduced into thefluid passage holes 21 b and 22 b, by plasma from propagating to theoutside over the protective covers 51. In more detail, each of theprotective covers 51 of the explosion proof device 5 includes a metalmesh 511 disposed at the outside of the pair of the electrodes 21 and22, and the metal mesh 511 has a wire diameter of 1.5 mm or less and anaperture ratio of 30% or more.

Hereinafter, usage of the plasma generation apparatus 100 in accordancewith this embodiment will be described. Optimization of the shape of theelectrodes to achieve both sterilization and deodorization is achievedby air ion measurement and ozone concentration measurement. Thesemeasurements are carried out at positions separated from the plasmaelectrode unit 2 in the downstream direction by distances at which anair ion counter may be installed. In this embodiment, a suction hole tomeasure ozone concentration is installed at a position separated fromthe plasma electrode unit 2 by 1 cm and a suction hole to measureionized water density is installed at a position separated from theplasma electrode unit 2 by 10 cm. Air ion measurement is an indirect andsimple measurement method, and is used to measure ions having chargesand a long life from among active species generated from plasma. The airion measurement uses relations between density of ionized water in airand density of active species under a regular plasma generationcondition. That is, a high density of the ionized water means a highdensity of active species effective in sterilization and deodorization.Since ozone, which is a by-product of plasma, has a much longer life(about 10 minutes or more) than ions, there is little difference betweenconcentration of ozone adjacent to plasma and concentration of ozone ata downstream point separated from the plasma. However, in order toincrease the absolute value of a measured value and check smallvariations of an amount of generated ozone, a sampling suction hole of ameasuring instrument is installed at a position separated from theelectrode 21 in the downstream direction by 1 cm.

An increase in the amount of generated ions due to the nonsymmetricalstructure of electrodes and half opening parts may be confirmed, asbelow.

Three kinds of electrodes having the same aperture ratio are prepared asin the following:

1) an electrode provided with only symmetrical complete opening parts(i.e., the fluid passage holes 21 b and the fluid passage holes 22having the same shape),

2) an electrode provided with nonsymmetrical complete opening parts, and

3) an electrode provided with half opening parts in addition tosymmetrical complete opening parts.

Voltage applied to the respective electrodes is adjusted to make theconcentration of ozone uniform, and densities of ionized water generatedunder the conditions are measured. Thereafter, the sums of thecircumferential lengths of the opening parts of the electrodes arecalculated, and ionized water densities per unit circumferential lengthare calculated by the measured ionized water densities. An increasedamount of generated ions due to the nonsymmetrical complete openingparts is obtained by comparing the electrodes 1) and 2) to each other,and an increased amount of generated ions due to the half opening partsis obtained by subtracting the ionized water density of the electrode 1)from the ionized water density of the electrode 3). FIG. 7 is a graphillustrating a ratio of an amount of generated ions of thenonsymmetrical complete opening parts to the symmetrical completeopening parts. As shown in FIG. 7, it is proved that the amount ofgenerated ions of the nonsymmetrical complete opening parts inaccordance with this embodiment increases 2 times or more, as comparedwith the symmetrical complete opening parts. Further, FIG. 8 is a graphillustrating a ratio of an amount of generated ions of the half openingparts to the symmetrical complete opening parts. As shown in FIG. 8, itis proved that the amount of generated ions of the half opening parts inaccordance with this embodiment increases 3 times or more, as comparedwith the symmetrical complete opening parts.

The plasma generation apparatus needs to suppress generation of ozoneharmful to human bodies as well as to increase the number of activespecies contained in a fluid contacting the fluid passage holes. Throughthe method in accordance with the embodiment, an amount of generatedactive species may be increased by decreasing a pulse width of a highvoltage pulse. Pulse width dependency of ionized water density and ozoneconcentration shown in FIG. 9 is measured when only a pulse width ischanged while allowing a repetition frequency and a peak voltage valueof a pulse to be uniformly maintained at 1 k. As shown in FIG. 9,ionized water is measured, an ozone concentration is lowered, and apulse width is decreased, at a pulse width of approximately 100 μs orless, and thus the ionized water is increased and the ozoneconcentration is decreased. Consequently, the ozone concentration may besuppressed to a low concentration and the ionized water density may beincreased. Therefore, it is understood that the pulse width isapproximately 100 μs or less.

Performance evaluation of the plasma generation apparatus in accordancewith this embodiment and the conventional plasma generation apparatus isexecuted through measurement of air ions in a high humidity condition.FIG. 10 is a graph illustrating results of measurement of ionized waterdensities when relative humidity is changed. From FIG. 10, in the caseof the plasma generation apparatus in accordance with this embodiment,it is understood that the ionized water density is decreased around ahumidity of approximately 75% RH, and is rapidly increased at a highhumidity of approximately 90˜100% RH, and a density of generated activespecies is high in the high humidity condition. On the other hand, inthe case of the conventional plasma generation apparatus, the ionizedwater density is decreased in proportion to increase of humidity, and israpidly decreased at a high humidity of approximately 90˜100% RH.Further, the plasma generation apparatus in accordance with thisembodiment and the conventional plasma generation apparatus havedifferent ionized water density ranges because the plasma generationapparatus in accordance with this embodiment and the conventional plasmageneration apparatus have different sizes of main bodies, or generatedifferent active species.

Further, a sterilization test of colon bacilli is carried out using theplasma generation apparatus in accordance with this embodiment. As aresult of operation of the plasma generation apparatus in a chamberhaving a volume of approximately 100 L for 6 hours under the conditionthat a laboratory dish inserted into an agar medium containing colonbacilli is placed in the chamber and humidity is maintained atapproximately 90% RH, colonies derived from bacilli are reduced and maybe sterilized, as shown in photographs of FIG. 11. Since colon bacilliare suppressed in the humidity condition having a small number of ions,as described above, it is understood that colon bacilli may also besterilized in the high humidity area having a large number of ions.

Deodorization executed around electrodes will be described, as below.First, a difference between concentration of active species generated byplasma and concentration of odors conveyed by an air current isconsidered. As shown in FIG. 12, a portion of plasma generated in a gapbetween the respective dielectric films 21 a and 22 a on the surfaces ofthe electrodes 21 and 22 is diffused into the fluid passage holes, andthus the generated active species interact with the air current suppliedfrom the air blowing device. Since an electron density of plasmagenerated from a space interposed between the dielectric films 21 a and22 a at the atmospheric pressure is about 1015/cm³ and a density ofgenerated ions or radicals is the same as the electron density, activespecies of a considerably high density are present. Further, a molecularnumber density supposed when molecules of an odor material are conveyedis approximately 1013/cm³ even in units of ppm, and has a smaller numberof ciphers than the density of the active species. That is, a space, inwhich a number of active species sufficient to decompose odor moleculesare generated, is formed within the fluid passage hole, and researchinto methods of transferring the active species to deodorization fields,as shown in FIG. 12, to promote decomposition of the odor molecules iscrucial. From among the methods, there is a method in which forced airis transferred to the fluid passage hole. Odor molecules contact thefluid passage hole provided with the deodorization fields by forciblytransferring the air to the fluid passage hole, thus being decomposed.There is another method in which odor molecules are transferred to thefluid passage hole by forced air generated by rotating a cylindricalmember at a position adjacent to the front surfaces of electrodes at ahigh speed, and are then decomposed in the deodorization fields.

Next, sterilization executed on the surface of an object separated fromthe electrodes will be described. Sterilization efficiency is determinedby a difference between a density of discharged active species and adensity of attached bacteria. As shown in FIG. 13, active species aregenerated by plasma charge, or are mixed with steam or fine droplets ofwater contained in an air current, are discharged to the outside of theapparatus as functional mist, and are then returned to stable moleculesvia recombination, etc., according to lives of the respective activespecies. In general, such ions present in air are measured by an air ionmeasuring instrument, and have a density of about 106/cm³ around plasma.When the active species, as functional mist, are diffused at such a lownumber density, a long time is required to decompose odor molecules, andthus, deodorizing effects are not expected, but the active species areeffective in sterilization of attached bacteria having a lower numberdensity. There are hundreds to thousands of attached bacteria per unitarea, i.e., approximately 102˜103/cm², and the attached bacteriacontinuously contact the active species in the functional mist for adesignated time, and are thus sterilized.

The explosion proof device 5 is required, for example, if the apparatusin accordance with this embodiment is installed in a refrigerator usinga combustible refrigerant. As shown in FIG. 14, even if the metal meshes511 are disposed around the plasma electrode unit 2 and a spark, such asarc discharge, is generated on the electrodes and ignites in acombustible refrigerant atmosphere, flame is not diffused to theentirety of the refrigerator over the metal meshes 511. Particularly, asshown in FIG. 15, when the metal meshes 511 have a wire diameter ofapproximately 1.5 mm or less or an aperture ratio of approximately 30%or more, the apparatus may be operated without loss of the amount ofgenerated active species increased due to the above electrode structure,i.e., may obtain safety without lowering deodorizing and sterilizingcapacities.

In the above-described plasma generation apparatus 100 in accordancewith this embodiment, a contact area between plasma generated from therespective corresponding fluid passage holes and a fluid is increased,thereby increasing an amount of generated active species. Further, steamor fine droplets of water contact the active species generated from thefluid passage holes, and thus the active species charge or are mixedwith the steam or fine droplets of water, become functional mist, andare then discharged to the outside, thereby sterilizing flowing bacteriaand attached bacteria. Moreover, the amount of the active speciesgenerated from plasma is increased, thereby exhibiting a sufficientdeodorizing function.

The disclosure is not limited to the above embodiment of the presentdisclosure.

As a non-limiting example, although the above embodiment illustrates thefine droplet supply device, a steam supply device may be provided. Inthis case, as the steam supply device, a steam generator may beinstalled instead of the mist generator. Further, the steam supplydevice may include a water supply unit to supply water to at least oneelectrode among a pair of electrodes to attach moisture to theelectrode, and a heating unit to evaporate the moisture attached to theelectrode. In this case, as the heating unit, a heater may be providedseparately from the electrodes, or heat generated during generation ofplasma from the electrode unit may be used. Further, both the steamsupply device and the fine droplet supply device may be provided.

As shown in FIG. 16, a heating unit 23 is provided on at least oneelectrode among a pair of electrodes and heats the electrodes toevaporate moisture and dry the electrodes. In this case, an electricheating wire, such as a heating resistor provided on the surface of theat least one electrode opposite to the opposite surface, may be used asthe heating unit 23, as shown in FIG. 16. Thereby, evaporated moisturemay become steam or fine droplets of water and be used as a raw materialof functional mist as well as an area in which plasma is generated maybe maximized and a large amount of generated active species may bemaintained.

Further, a temperature sensor may be provided on at least one electrodeamong a pair of electrodes such that steam or fine droplets of water aresupplied when a temperature detected by the temperature sensor is morethan a designated value. In this case, a control device acquires adetection signal from the temperature sensor and compares a temperatureindicated by the detection signal with a predetermined value,simultaneously, and outputs a mist generation signal to the mistgenerator upon judging that the temperature of the electrodes is morethan the designated value. Then, the mist generator acquires the mistgeneration signal and supplies mist toward the plasma electrode unit. Inaddition, the control device may be configured to execute feedbackcontrol of an amount of supplied mist or steam based on the temperatureof the electrodes.

Further, if the temperature sensor to measure the temperature of atleast one electrode, among a pair of electrodes, and the heating unit toheat at least one electrode, among the pair of electrodes, may beprovided, the heating unit may be configured to heat the electrodes ifthe temperature detected by the temperature sensor is less than adesignated value. Further, the control device acquires a detectionsignal from the temperature sensor and compares a temperature indicatedby the detection signal with a predetermined value, simultaneously, andsupplies current to the heating unit, i.e., the heating resistor, uponjudging that the temperature of the electrodes is less than thedesignated value. Thereby, the electrode unit is heated to evaporatewater drops attached to the electrode unit, and functional mist isgenerated using generated steam. In addition, the control device may beconfigured to execute feedback control of a heating temperature of theheating unit based on the temperature of the electrodes.

Further, a humidity sensor to measure a relative humidity in theatmosphere between a pair of electrodes may be provided to supply steamor fine droplets of water when the humidity detected by the humiditysensor is less than a designated value. In this case, the control deviceacquires a detection signal from the humidity sensor and compares ahumidity indicated by the detection signal with a predetermined value,simultaneously, and outputs a mist generation signal to the mistgenerator upon judging that the relative humidity in the atmosphere isless than the designated value (for example, approximately 90% RH).Then, the mist generator acquires the mist generation signal andsupplies mist toward the plasma electrode unit. In addition, the controldevice may be configured to execute feedback control of an amount ofsupplied mist or steam based on the relative humidity.

Although this embodiment illustrates the plural fluid passage holes 21 bof the electrode 21 as having the same shape and the plural fluidpassage holes 22 b of the electrode 22 as having the same shape, theplural fluid passage holes 21 b and the plural fluid passage holes 22 bmay have different shapes.

Further, although this embodiment illustrates all of the fluid passageholes 21 b of the electrode 21 as being larger or smaller than all ofthe fluid passage holes 22 b of the electrode 22, some of the fluidpassage holes 21 b of the electrode 21 may be smaller than the fluidpassage holes 22 b of the electrode 22 and the remaining fluid passageholes 21 b may be larger than the fluid passage holes 22 b of theelectrode 22.

Further, although this embodiment illustrates the through holes as beingformed on either of the electrodes 21 and 22, the through holes (halfopening parts) may be formed on both the electrodes 21 and 22.

Further, although this embodiment illustrates the fluid passage holes 21b and 22 b as having cross-sections having designated diameters, thefluid passage holes 21 b and 22 b formed on the respective electrodes 21and 22 may have various shapes, such as a shape having a tapered plane,a mortar shape or a bowl shape, i.e., a shape having a diameterdecreased or increased from one opening to the other opening.

Further, the fluid passage holes 21 b and 22 b may have various shapesother than a circle, i.e., an oval, a rectangle, a rectilinear slit, aconcentric slit, a wave-shaped slit, a crescent moon, a comb, ahoneycomb, or a star.

As is apparent from the above description, a plasma generation apparatusand method in accordance with example embodiments of the presentdisclosure achieve both sterilization and deodorization of attachedbacteria even under the condition that steam or fine droplets of waterare present.

Although a few embodiments of the present disclosure have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in these embodiments without departing from theprinciples and spirit of the disclosure, the scope of which is definedin the claims and their equivalents.

What is claimed is:
 1. A plasma generation method, the methodcomprising: preparing a pair of electrodes; and applying a designatedvoltage between the pair of electrodes to carry out plasma discharge,wherein fluid passage holes are provided at corresponding parts ofrespective electrodes so as to communicate with each other, and steam orfine droplets of water are applied to the fluid passage holes and plasmagenerated around the fluid passage holes.
 2. The plasma generationmethod of claim 1, wherein the fluid passage holes of each respectiveelectrode, among the pair of electrodes, are concentric with the fluidpassage holes of the other electrode.
 3. A plasma generation method, themethod comprising: preparing a pair of electrodes; and applying adesignated voltage between the pair of electrodes, wherein through holesare provided on one electrode such that openings of the through holes onan opposite surface of the electrode facing the other electrode isclosed by the other electrode, and steam or fine droplets of water areapplied to openings of the through holes on the other surface of theelectrode and plasma generated around the through holes.
 4. The plasmageneration method of claim 3, wherein the fine droplets of water have aparticle size of 100 μm or less.
 5. The plasma generation method ofclaim 4, wherein voltage in a pulse mode is applied to each respectiveelectrode and has a peak value within a range of 100V to 5,000V and apulse width within a range of 0.1 μs to 300 μs.
 6. The plasma generationmethod of claim 5, wherein the steam or fine droplets of water aresupplied when a temperature of the pair of electrodes is more than adesignated value.
 7. The plasma generation method of claim 6, whereinthe pair of electrodes is heated by a heating unit when the temperatureof the pair of electrodes is less than the designated value.